Radio frequency signal transmitter arrangement



1966 w. A. SCHANBACHER 3,270,284

RADIO FREQUENCY SIGNAL TRANSMITTER ARRANGEMENT Filed Jan. 13, 1964 \4 54 f 55 7o 66 64 74 Zy- 1 -WT. ANTENNA MREAAKIES 6O 72 I I I I I I I I T 86 82 5% 4 INVENTOR.

W/LL/A/M A. SCHA/VBAMER WM @2 av m A 7TO 2NE V 5 IGNAL CURRENT United States Patent 3,270,284 RADIO FREQUENCY SIGNAL TRANSMITTER ARRANGEMENT William A. Schanbacher, 7200 W. 90th St., Los Angeles, Calif. Filed Jan. 13, 1964, Ser. No. 337,354 11 Claims. (Cl. 325-105) This invention relates to the transmitter art and more particularly to an improved compact, low Weight, transistorized RF transmitter incorporating an audio frequency amplitude modulation of an RF signal.

In many remotely controlled actuator applications such as automatic garage door openers and the like, it has long been desirable to provide a light-weight, compact radio frequency (RF) signal transmitter. In such an application the transmitter is generally carried in a motor vehicle and transmits a coded signal. A tuned receiver associated with an actuator receives the signal and initiates operation of the actuator to either open or close the garage door. Further, it is often desirable that such transmitters be able to transmit a coded RF signal so that a plurality of such actuators may be utilized in a comparatively small geographical area, such as neighboring houses, wherein the radiated signal will selectively initiate operation of only one of many similarly equipped garage doors. Therefore, the transmitter for such an application should be highly stable, that is, very little signal frequency drift over extended periods of time and over a Wide range of ambient temperatures.

As the transmitter is generally carried in a motor vehicle, it is preferably light in weight and compact in size. Also, a low cost, readily available power source such, as, for example, standard dry cell batteries, are preferably utilized to provide an integral internal and easily replaceable power source, rather than power the transmitter directly from connections to the motor vehicle battery. In this arrangement, failures of the motor vehicle battery do not prevent operation of the transmitter and initiation of the operation of the actuators may be carried on regardless of motor vehicle battery condition. The transmitter itself, of course, should preferably have a low power drain in order to extend the life of such dry cell batteries.

The transmitter is generally only required to transmit intermittently a signal over comparatively short distances, for example, 300 yards, and thus the radiated power may be comparatively low, for example, on the order of milliwatts. The transmitter should preferably be transistorized so that the power drain is minimal.

To provide the coding of the radiated signal it has been found desirable that the frequency of the radiated RF signal be variable, within limits, such as between 250 and 300 megacycles, and, also, that the radiated signal be amplitude modulated by an audio frequency signal. The frequency of the audio frequency amplitude modulation signal is preferably controllable also and within the range of, for example, 12 to 24 kilocycles. By judicious selection of the particular RF signal frequency and the particul-ar'audio signal frequency a comparatively large number of such transmitters may be utilized in a comparatively small geographic area with complete selectivity so that any one transmitter radiates only a signal that initiates operation of a single actuator through reception thereof by a receiver tuned to receive the particular coded, audio frequency amplitude modulated RF signal.

Accordingly, it is an object of applicants invention to provide an improved transmitter.

It is another object of applicants invention to provide a highly stable RF transmitter in which the radiated RF signal is audio frequency modulated at a pre-selected audio frequency rate.

3,270,284 Patented August 30, 1966 It is a further object of applicants invention to provide a low voltage, low current drain transmitter in which the radiated RF signal may be coded by preselecting a particular RF frequency and amplitude modulating this RF frequency with a preselected audio frequency signal.

The above and other objects are achieved, according to one embodiment of applicants invention, by providing an RF transmitter comprised of substantially two sections: an audio frequency signal generating section and an RF signal generating and transmitting section. Since it is desirable to utilize a comparatively low voltage power supply as, for example, low voltage dry cell batteries, applicant preferably utilizes transistors in his improved RF transmitter to minimize power drain.

In a prefered embodiment of applicants invention an audio frequency oscillator, such as Hartley oscillator, is coupled between the base and emitter electrodes of an audio frequency switching transistor to provide an audio frequency signal at a pre-selected frequency at the collector electrode thereof. This collector electrode of the audio frequency switching transistor is coupled to the emitter electrode of a power RF oscillator transistor and feeds the audio frequency signal therein. The audio frequency switching transistor and the audio frequency oscillator comprise the audio frequency signal generating portion of applicants improved RF transmitter.

An RF tank circuit is coupled between the base electrode and the collector electrode of the power RF oscillator transistor and the RF tank circuit includes a tapped inductor and a variable capacitor for controlling the frequency of a generated RF signal. An end fed antenna is connected in series with an inductor to the emitter elec trode of the power RF oscillator transistor and the inductor has an impedance which, together with the impedance of the tapped inductor of the RF tank circuit, substantially matches the resistive impedance of the power RF oscillator transistor. The inductor is connected between the input to the antenna and the tapped inductor of the RF tank circuit.

Applicant prefers that the impedance into the end fed antenna be the above described inductance matching the resistive impedance of the power R-F oscillator transistor and that it be sufiiciently reactive so that changes in the antenna radiation impedance due to, for example, the proximity of foreign objects, do not significantly effect the phase angle of the load presented by the antenna. Thus, the antenna is maintained in the RF signal generating circuit loop and the RF signal generating and transmitting section comprises the above described power RF oscillator transistor, RF tank circuit, end fed antenna, inductor and bias resistor coupled to the power RF oscillator transistor base electrode,-and is preferably substantially isolated from the audio frequency signal generating portion of applicants improved transmitter by a coupling capacitor that is coupled between the antenna termination and the emitter electrode of the power RF oscillator transistor.

Since transistors are basically current controlling devices, the coupling of the audio frequency switching transistor to the power RF oscillator transistor as above described applies a substantially square wave audio frequency amplitude current modulation by the generated audio frequency signal to the generated RF signal so that the antenna radiates a substantially square wave signal.

A low voltage power source such as, for example, a 22 /2 volt battery, may be utilized to energize the transmitter.

Applicant has found that, in such an arrangement, the radiated RF signal has a comparatively narrow bandwidth. For example, on the order of kilocycles, as compared with the more usual 8 to 10 megacycles in the radiated RF signals from prior art transmitters. This provides that substantially larger number of actuators dependent upon receipt of such an RF signal for initiatin'g actuation may be utilized in a given geographic area than has heretofore been achieved. Applicant has also found that providing a rake means coupled to the antenna, the radiating area of applicants transmitter is substantially increased to provide greater area coverage for his radiated signal.

In the preferred form of applicants invention, applicant utilizes a circuit board to provide connections between the various components of the transmitter and the conductive elements of the circuit board can also provide some of the elements themselves such as, for example, the antenna and the tapped inductor of the RF tank circuit in the RF signal generating and transmitting section of the transmitter.

The above and other embodiments of applicants invention are more fully described in the following detailed description taken together with the accompanying drawings wherein similar reference characters refer to similar elements throughout and in which:

FIGURE 1 is a schematic diagram of applicants improved transmitter;

FIGURE 2 is a graphic representation of the signals generated in the embodiment shown in FIGURE 1;

FIGURES 3 and 4 illustrate an embodiment of applicants invention.

Referring now to FIGURE 1, there is shown a schematic diagram for one embodiment of applicants invention. As shown thereon a transmitter, generally designated 10, may be considered to be comprised of two sections: an audio frequency signal generating section 12 and an RF signal generating and radiating section 14. The audio frequency signal generating section 12 comprises an audio frequency oscillator 16 and an audio frequency switching transistor 18. In the preferred embodiment of applicants invention applicant utilizes a Hartley oscillator as the audio frequency oscillation oscillator 16 since the Hartley oscillator is highly degenerate and has a comparatively low power drain.

The well known Hartley oscillator comprises resistors 20, 2 2, 2 4 and 26 coupled to a transistor 28 as shown in FIGURE 1, an audio frequency tank circuit 2 9 including a capacitor and variable inductor 32, and a load resistor 34. The Hartley oscillator or any other audio frequency oscillator 16 that may be utilized in applicants invention herein is coupled between the base electrode 36 and the emitter electrode 38 of the audio frequency switching transistor 18 to provide a generated audio frequency signal between the emitter electrode 3 8 and the collector electrode 52 of the audio frequency switching transistor 18.

The preselected frequency of the generated audio freque-ncy signal is obtained by varying the inductance of the variable inductor 32 in the audio frequency tank circuit 29.

In the preferred embodiment of applicants invention a low voltage power source 40, such as a 22 /2 volt dry cell battery, has its plus voltage side connected through a momentary contact switch 412 into the audio frequency signal generating section 12 for connection to the emitter electrode 38 of the audio frequency switching transistor 18. The negative side of the battery is ground and, therefore, the transistors utilized in the embodiment shown on FIGURE 1 are of the P-N-P type so that correct polarity thereon is maintained. Similarly, if desired, the positive side of the battery 40 may be ground and, in this event, the transistors utilized should be of the N-P-N type for correct polarity.

When utilizing the Hartley oscillator as shown on FIGURE 1, applicant has found that the components thereof preferably have the following values: resistor 20220,000 ohms, resistor 2210,000 ohms, resistor 24 4,700 ohms, resistor 26100(l ohms, transistor 28a Texas Instrument Corporation transistor No. 2N404, or

an RCA transistor 2N414, capacitor 30.0075 microfarad, inductor 32variable 5 to 20 millihenries, and resistance 3433,000 ohms. The audio frequency switching transistor 18 is also preferably a Texas Instrument 2N404 or RCA 2N4l4 transistor. This circuit generates the preferred audio frequency signal between the emitter electrode 30 and collector electrode 52 at the audio frequency switching transistor 18 in the range of 12 to 24 kilocycles, depending upon the particular value of the inductor 32 that is utilized in the audio frequency tank circuit 29.

In the RF signal generating and transmitting section 14 of the transmitter 10 there is a power RF oscillator transistor 44 having a base electrode 46, a collector electrocle 48, and an emitter electrode 50. The emitter electrode 50 of the pow-er RF oscillator transistor 44 is connected to the collector electrode 52 of the audio frequency switching transistor .18. This feeds the generated audio frequency signal generated in the audio frequency signal generating section 12 into the RF signal generating and transmitting section 14 for modulation of an RF signal generated therein.

An RF tank circuit 54 comprises a parallel connected variable capacitor 56 and tapped inductor 58, series connected with capacitor 60 between the base electrode 46 and the collector electrode 48 of the power RF oscillator transistor 44. By varying the capacitance of the variable capacitor 56 the particular RF frequency of the generated RF signal may be selected.

Applicant has found that the reactive impedance of capacitor 60 tends to minimize the phase shift, and resulting frequency shift, of the generated RF signal that occurs due to temperature changes of the power RF oscillator transistor 44 inducing changes in the current gain thereof. Thus, in the preferred embodiment of applicants invention the capacitor 60 is included for temperature compensation to improve the RF signal frequency' stability.

An end fed antenna 64 is provided between the tank circuit 54 and the emitter electrode 50 of the power RF oscillator transistor 44 for radiating an RF signal and has an input terminal 66 and an end terminal 68. The input terminal 66 of the end fed antenna 64 is coupled by an inductor 70 to the tapped inductor 58 of the RF tank circuit 54. In the preferred embodiment of applicants invention the impedance of the inductor 70, together with the impedance of the tapped inductor 58 substantially matches the resistive impedance of the power RF oscillator transistor 44.

The output impedance of the power RF oscillator tran sistor 44 varies somewhat from transistor to transistor. Applicant has found it advantageous in the production of his transmitter 10 to measure the output impedance of the power RF oscillator transistor 44 and provide a fixed inductance value of the tapped inductor 58 in the tank circuit 54. The exact value of the inductance of the inductor 70 is then selected so that the impedance of power RF oscillator transistor 44 is matched by the sum of the impedance of the tapped inductor 70. This impedance match tends to provide a more eflicient opera tion of the transmitter 10 in that the radiated power in the signal radiated by the antenna 64 is maximized for a given energy input.

Further, the above described impedance matching of the resistive output impedance of the power RF oscillator transistor 44 by the tapped inductor 58 of the RF tank circuit 54 and the inductor 70 provides a high de gree of RF signal stability since changes in the radiating impedance of the antenna 64 are substantially isolated, as regards RF signal phase shift to the RF tank circuit which, if it were present, would tend to vary the frequency of the RF signal from the pre-selected value.

A bias resistor 72 connects the base electrode 46 'of the power RF oscillator transistor 44 to ground for direct current biasing and filtering, as describedbelow.

The bias resistor 72 is preferably included to provide this predetermined direct current into the base electrode 46 of power RF oscillator transistor 44 so that the desired radiated power and signal wave shape is achieved.

Thus, an RF signal having a pre-selected frequency determined primarily by the value of the variable capacitor 56 is the RF tank circuit 54 is generated between the emitter electrode 50 and collector electrode 48 of the power RF oscillator transistor 44. This RF signal is square wave amplitude current modulated by the audio frequency signal applied to the power RF oscillator transistor 44 from the audio frequency switching transistor 18 as described below in more detail in the discussion of FIGURE 2.

If desired, a rake means 74 may be coupled to and made a part of the end fed antenna 64 to provide an increase in the radiating area thereof. By the term rake means applicant means an addition to the antenna for the function of increasing the area available for radiation of the square wave current amplitude modulated RF signal. The rake means 74 may take the form of a serpentine type curved electrical conductor coupled to the antenna 64 as shown on FIGURES 3 and 4.

As shown in FIGURE 1, in the preferred embodiment of applicants invention the antenna 64 is in the loop of the RF signal generating and transmitting portion of the transmitter 10.

In the preferred embodiment of applicants invention the power RF oscillator transistor 44 is a 2N2363 transistor such as Texas Instrument Corporation No. T1390. Variable capacitor 56 is a capacitor having a range of 1.5 to 7 micromicrofarads, capacitor 60 is 3.3 micromicrofarads, and inductance 58 is .06 microhenry. The inductance 70 preferably has a value of approximately 0.2 microhenry when utilized with the above described 2N23 63 power RF oscillator transistor 44, the exact value depending on the output impedance thereof. Bias resistor 72 has a value of 220,000 ohms.

To substantially isolate the RF signal generating and transmitting section 14 from the audio frequency signal generating section 12 and the power supply 40, applicant prefers to provide a coupling capacitor 76, which, for example, might have a value of .001 microfarad, between the end fed antenna 64 end terminal 68 and the connection of the emitter electrode 50 of the power RF oscillator transistor 44 and the collector electrode 52 of the audio frequency switching transistor 18. This substantially isolates the two sections.

As noted above, transistors are essentially current control devices and thus, in the circuit shown on FIGURE 1, the audio frequency signal applies a substantially square wave current amplitude modulation to the RF signal.

FIGURE 2 is a graph illustrating the current of the output wave form at the radiated signal from antenna 64.

As shown on FIGURE 2 curve a represents the RF signal and curve b represents the audio frequency amplitude modulation envelope.

Portion b of curve b is the rise time of the audio frequency modulation envolep curve I) and is determined by the loop gain of the RF signal generating and transmitting section 14 and the Q of the RF tank circuit 54.

The conductive time b of the curve b is the conductive time of the power RF oscillator transistor 44 and is determined by the conducting time of the audio frequency switching transistor 18 which feeds the audio frequency signal into the power RF oscillator transistor 44 and provides a substantially flat wave shape illustrated in this portion by the self-limiting action of the diode elfect between collector 48 and base 46 of the power RF oscillator transistor 44, together with the filtering effect of capacitor 60 and bias resistor 72.

The non-conductive time b of the curve 11 is the nonconductive time of the power RF oscillator transistor 44 and is controlled by the non-conductive time of the audio frequency switching transistor 18.

As noted above, applicant prefers to provide his transmitter 10 of FIGURE 1 in a compact, low weight configuration. To achieve these objects applicant prefers that the components comprising the transmitter 10 be mounted on a circuit board with conductive elements on the circuit board providing the connection between the components and the circuit board mounted in, for example, a plastic case 79, of FIGURE 3.

FIGURE 3 illustrates the radio frequency generating and transmitting section 14 as embodied on a circuit board. The circuit board comprises an insulating sheet having a first face 82 on which conductive elements are mounted and an opposite face 84 on which the components are mounted. Contact of the components with the conductive elements is made through the insulating sheet 80 in conventional fashion. FIGURE 3 illustrates the face 82 of the insulating sheet 80 upon which the conductive strips are mounted. The actual components are therefore shown in dotted line as they are on the opposite face 84. The conductive strip 86 actually comprises the antenna 64, and, as shown in FIGURE 3, the rake means 74 is coupled thereto intermediate the input terminal 66 and the end terminal 68 of the antenna 64. This is illustrated in detail on FIG- URE 4 which shows the connections of the rake means 74 through the insulating sheet 80 to the conductive element 86 comprising the antenna 64. It will be appreciated that the serpentine curved rake means 74 is only one shape that the rake means 74 may take. Any other shape electrically conductive member that effectively increases the radiating area of the antenna 64 may be considered a rake means within applicants invention.

The inductor 58 of the radio frequency tank circuit 54 is also provided by a section of conductive element 90. Thus, it can be seen that for economy in manufacturing it is preferred that the location 92 of the tap into the tapped inductor 58 be maintained constant so that all circuit boards will have substantially the same configuration. The inductance of the inductor 70 is then varied to achieve the above described matching to the resistive impedance output of the power RF oscillator transistor 44.

I claim:

1. An improved low power, audio frequency amplitude modulated RF transmitter of the kind having an audio frequency oscillator for applying an audio frequency square wave amplitude current modulation to a radio frequency signal comprising in combination:

a low voltage powered Hartley oscillator;

an audio frequency switching transistor having base,

emitter and collector electrodes;

a load resistor coupled to said base electrode of said audio frequency switching transistor, and said Hartley oscillator coupled between said load resistor and said emitter electrode of said audio frequency switching transistor for generating an audio frequency switching transistor for generating an audio frequency signal between said emitter and said collector electrodes of said audio frequency switching transistor;

an RF signal generating and transmitting circuit comprising a power RF oscillator transistor having base, emitter and collector electrodes, a variable capacitance tank circuit coupled between said base electrode and said collector electrode of said power RF oscillator transistor for controlling the frequency of an RF signal, an antenna having an input terminal and an end terminal for radiating an RF signal, an inductor coupled between said tank circuit and said input terminal of said antenna and the inductance of said tank circuit and the inductance of said inductor together matching the impedance of said power RF oscillator transistor, and a bias resistor coupled to said base electrode of said power RF oscillator transistor, whereby an RF signal is generated between said emitter electrode and said collector electrode of said power RF oscillator transistor;

and said emitter electrode of said power RF oscillator transistor coupled to said collector electrode of said audio frequency switching transistor for receiving said audio frequency modulation signal to square wave current amplitude modulate said RF signal;

a coupling capacitor for isolating said RF signal generating and transmitting circuit from said Hartley oscillator coupled between said end terminal of said antenna and said emitter electrode of said power RF oscillator transistor;

and means for energizing said Hartley oscillator and said RF signal generating and transmitting circuit.

2. The arrangement defined in claim 1 and further comprising a rake means coupled to said antenna for increasing the signal radiating area thereof, and the frequency of said audio frequency signal is in the range of 12 to 24 kilocycles, and the frequency of said RF signal is in the range of 250 to 300 megacycles with a band width of approximately 100 kilocycles.

3. An RF signal transmitter comprising in combination:

an audio frequency switching transistor having base,

emitter and collector electrodes;

a power RF oscillator transistor having base, emitter and collector electrodes and the emitter electrode of said power RF oscillator transistor coupled to the collector electrode of said audio frequency switching transistor;

an RF tank circuit having a first preselected inductance, for controlling the frequency of an RF signal, coupled between said base electrode and said collector electrode of said power RF oscillator transistor;

an inductor, an antenna, and a coupling capacitor connected in series between said tank circuit and said emitter electrode of said power RF oscillator transistor, and said inductor having a second preselected inductance and the sum of said first preselected inductance and said second preselected inductance substantially matching the impedance of said power RF oscillator transistor to maintain an inductive coupling into said antenna whereby an RF signal is generated between said emitter and said collector electrodes of said power RF oscillator transistor;

an audio frequency oscillator coupled between said base and said emitter electrode of said audio frequency switching transistor for generating an audio frequency modulation signal between said emitter and said collector electrode of said audio frequency switching transistor;

a bias resistor connected to said base elecrode of said power RF oscillator transistor;

and means for energizing said audio frequency oscillator and said power RF oscillator transistor whereby said RF signal is square wave current amplitude modulated by said audio frequency signal.

4. An RF signal transmitter comprising in combination:

audio frequency signal generating means for generating an audio frequency signal having a first preselected frequency;

RF signal generating means for generating an RF signal having a frequency within a second preselected frequency range different from said first preselected frequency range;

said RF signal generating means comprising a power RF oscillator transistor having base, emitter and collector electrodes for providing said RF signal between said emitter and said collector electrodes of said power RF oscillator transistor;

said audio frequency generating means comprising an audio frequency switching transistor having base, emitter and collector electrodes for providing said audio frequency signal between said emitter and said collector electrodes of said audio frequency switching transistor, and said collector electrode of said audio frequency switching transistor coupled to said emitter electrode of said power RF oscillator transistor to square wave current amplitude modulate said RF signal at said audio frequency;

and an antenna means in said RF signal generating means and coupled to said emitter electrode of said power RF oscillator transistor for transmitting said square wave current modulated RF signal.

5. An RF signal transmitter comprising in combination:

audio frequency generating means for generating an audio frequency signal having a first prselected frequency;

RF signal generating means for generating an RF signal having a frequency within a second preselected frequency range different from said first preselected frequency range;

said RF signal generating means comprising a power RF oscillator transistor having base, emitter and collector electrodes for providing said RF signal between said emitt-er and said collector electrodes of said power RF oscillator transistor;

said audio frequency signal generating means comprising an audio frequency switching transistor having base, emitter and collector electrodes for providing said audio frequency signal between said emitter and said collector electrodes of said audio frequency switching transistor, and said collector electrode of said audio frequency switching transistor coupled to said emitter electrode of said power RF oscillator transistor to square wave current amplitude modulate said RF signal at said audio frequency;

an antenna means coupled in said RF signal generating means for transmitting said square wave current modulated RF signal;

and a coupling capacitor coupled between said emitter electrode of said power RF oscillator transistor and said antenna means for substantially isolating said audio frequency generating means from said RF signal generating means.

6. The arrangement defined in claim 3 wherein the said RF tank circuit comprises a parallel connected variable capacitor and a tapped inductor having said first preselected inductance, and a temperature compensating capacitor, and said parallel connected variable capacitor and tapped inductor connected in series with said tem- 7. An RF signal transmitter comprising in combination:

a power RF oscillator transistor having base, emitter and collector electrodes;

a tank circuit comprising a variable capacitor and a tapped inductor having a first preselected inductance connected in parallel and a temperature compensating capacitor connected in series with said parallel connected variable capacitor and tapped inductor between said base electrode and said collector electrode of said power RF oscillator transistor for controlling the frequency of an RF signal;

an antenna having an input connection and an end connection;

an inductor having a second preselected inductance coupled to said input connection of said antenna and to said tapped inductor of said tank circuit and the sum of said first preselected inductance and said second preselected inductance providing an impedance substantially matching the impedance of said power RF oscillator transistor;

a bias resistor coupled to said base electrode of said power RF oscillator transistor to provide a predetermined direct current into the base electrode of said power RF oscillator transistor whereby an RF signal is generated at a preselected frequency between said emitter electrode and said collector electrode of said power RF oscillator transistor;

means for generating an audio frequency signal, said means connected to said emitter electrode of said power R-F oscillator transistor for providing said audio frequency signal therein to modulate said RF signal;

and a coupling capacitor coupled between said antenna end terminal and said emitter electrode of said power RF oscillator transistor for substantially isolating said means from said power RF oscillator transistor.

8. In combination:

an audio frequency oscillator;

an audio frequency switching transistor having base,

emitter and collector electrodes and said audio frequency oscillator coupled between said base and said emitter electrodes for generating an audio frequency modulation signal between said emitter and said collector electrodes of said audio frequency switching transistor;

a power RF oscillator transistor having an emitter electrode connected to said collector electrode of said audio frequency switching transistor for receiving said audio frequency signal, and a base and a collector electrode;

a tank circuit for controlling the frequency of an RF signal, said tank circuit comprising a variable capacitor and a tapped inductor connected in parallel, and a first side thereof connected to said collector electrode of said power RF oscillator transistor, and .a temperature compensating capacitor connected between said base electrode of said power RF oscillator transistor and said tank circuit, and said tapped inductor having a first preselected inductance;

an antenna for transmitting an RF signal and having an input connection and an end connection;

an induct-or having a second pre-se-lected inductance and the sum of said first preselected inductance and said second preselected inductance substantially matching the impedance of said power RF oscillator transistor, and said inductor connected between said tapped inductor of said tank circuit and said input terminal of said antenna;

and a bias resistor coupled to said base electrode of said power RF oscillator transistor whereby an RF signal is generated between said emitter and said col lector electrodes of said power RF oscillator transistor and said RF signal is square wave current amplitude modulated by said audio frequency signal.

9. In combination:

an audio frequency oscillator;

an audio frequency switching transistor having base,

emitter and collector electrodes and said audio frequency oscillator couple-d between said base and said emitter electrodes of said audio frequency switching transistor for generating an audio frequency signal between said emitter electrode and said collector electrode of said audio frequency switching transistor;

and an RF generating and transmitting circuit comprising a power RF oscillator transistor having base, emitter and collector electrodes and said emitter electrode of said power RF oscillator transistor connected to said collector electrode of said audio frequency switching transistor for receiving said audio frequency signal therein, and means connected between said base electrode and said collector electrode of said power RF oscillator transistor for providing an RF signal between said emitter electrode and said collector electrode of said power RF oscillator transistor whereby said audio frequency signal square wave current amplitude modulates said RF signal, and means for radiating said amplitude modulated RF signal.

10. A transmitter comprising in combination:

an audio frequency oscillator;

an audio frequency switching transistor having base,

emitter and collector electrodes and said audio frequency oscillator coupled between said base and said emitter electrodes of said audio frequency switching transistor for generating an audio frequency modulation signal having a preselected frequency between said emitter and said collector electrodes of said audio frequency switching transistor;

a power RF oscillator transistor having base, emitter and collector electrodes, said emitter electrode of said power RF oscillator transistor coupled to said collector electrode of said audio frequency switching transistor;

an end fed antenna having an input terminal and an end terminal for radiating an RF signal;

a tank circuit having a first preselected inductance for controlling the frequency of an RF signal, coupled between said collector and said base electrodes of said power RF oscillator transistor, an inductor having a second preselected inductance and the sum of said first and said second preselected inductances substantially matching the impedance of said power RF oscillator transistor, and said inductor coupled between said tank circuit and said input terminal of said antenna for feeding an RF signal into said antenna, a bias resistor coupled to said base electrode of said power RF oscillator transistor, and a rake means coupled to said antenna for increasing the radiation transmission area thereof, whereby an RF signal is generated between said emitter and said collector electrodes of said power RF oscillator transister;

a coupling capacitor coupled to the end terminal of said antenna and said emitter electrode of said power RF oscillator transistor for substantially isolating said audio frequency oscillator;

and a low voltage power source for energizing said audio frequency oscillator whereby said RF signal is square wave current modulated by said audio frequency signal.

11. The arrangement defined in claim 10 wherein said RF signal is radiated by said antenna at approximately 10 milowatts, said audio frequency signal has a frequency within the range of 12 to 24 kilocycles and said RF signal has a frequency within the range of 250 to 300 megacycles and a band width of approximately kilocyc es.

References Cited by the Examiner UNITED STATES PATENTS 3,068,415 12/1962 Johnson 325-104 3,108,223 10/1963 Hunter 325-111 X DAVID G. REDINBAUGH, Primary Examiner.

JOHN W. CALDWELL, Examiner. 

9. IN COMBINATION: AN AUDIO FREQUENCY OSCILLATOR; AN AUDIO FREQUENCY SWITCHING TRANSISTOR HAVING BASE, EMITTER AND COLLECTOR ELECTRODES AND SAID AUDIO FREQUENCY OSCILLATOR COUPLED BETWEEN SAID BASE AND SAID EMITTER ELECTRODES OF SAID AUDIO FREQUENCY SWITCHING TRANSISTOR FOR GENERATING AN AUDIO FREQUENCY SIGNAL BETWEEN SAID EMITTER ELECTRODE AND SAID COLLECTOR ELECTRODE OF SAID AUDIO FREQUENCY SWITCHING TRANSISTOR; AND AN RF GENERATING AND TRANSMITTING CIRCUIT COMPRISING A POWER RF OSCILLATOR TRANSISTOR HAVING BASE, EMITTER AND COLLECTOR ELECTRODES AND SAID EMITTER ELECTRODE OF SAID POWER RF OSCILLATOR TRANSISTOR CONNECTED TO SAID COLLECTOR ELECTRODE OF SAID AUDIO FREQUENCY SWITCHING TRANSISTOR FOR RECEIVING SAID AUDIO FREQUENCY SIGNAL THEREIN, AND MEANS CONNECTED BETWEEN SAID BASE ELECTRODE AND SAID CCOLLECTOR ELECTRODE OF SAID POWER RF OSCILLATOR TRANSISTOR FOR PROVIDING AN RF SIGNAL BETWEEN SAID EMITTER ELECTRODE AND SAID COLLECTOR ELECTRODE OF SAID POWER RF OSCILLATOR TRANSISTOR WHEREBY SAID AUDIO FREQUENCY SIGNAL SQUARE WAVE CURRENT AMPLITUDE MODULATES SAID RF SIGNAL, AND MEANS FOR RADIATING SAID AMPLITUDE MODULATED RF SIGNAL. 